Properties of nanocrystalline Ni/Al2O3 composites

Nanocrystalline Ni/Al2O3 composites were formed by electrolytic co-depositi on. The co-deposition was achieved by adding Al2O3 nanopowder having a medi an particle diameter of 13 nm into different electrolytic baths followed by direct current (DC) plating with current densities j up to 8 A/dm(2). The structure and phase distribution of Ni/Al2O3 compound film were investigate d after different heat treatments by light microscopy and in a transmission electron microscope (TEM) equipped with energy dispersive X-ray (EDX) unit . Beside that the incorporated ceramic nanoparticles are largely homogeneou sly distributed in the nickel matrix, it was found that they also act like an inhibitor for growth of nuclei resulting in significant increase of nucl eation rate of nickel grains on the growing composite film. Therefore, the adding of alumina nanopowder to the electrolytic bath can promote the forma tion of a nanocrystalline nickel matrix with an average matrix grain size o f less than 50 nm. whereas the same bath without nanopowder forms films of much larger grains even at larger j. Futhermore, it was found by TEM inspec tion that the nanocrystalline matrix structure is largely stabilised agains t grain growth even after long-term thermal treatments at 1050 degrees C. T his was also confirmed by measurements of the coercivity and microhardness. Even though these properties decreased after thermal treatment due to hydr ogen release from the composites, they showed constant enhanced values afte r further heat treatments, compared with pure nickel specimens.